Embodiments of the invention relate generally to induction motors having a low leakage inductance and, more particularly, to a method and system for designing low-inductance, high-efficiency induction machines by providing power converters that include silicon carbide metal-oxide-semiconductor field effect transistors (MOSFETs).
Induction machines are typically driven using a power converter with active silicon switching devices. The switching devices are typically operated using a pulse-width modulation (PWM) technique to convert a DC supply power to an AC power that can be used to drive the induction machine. In PWM, the switching devices are driven using a train of DC pulses. When a voltage is suddenly applied from the power converter across an inductive load, the current through the load rises almost linearly with time. When the voltage is then turned off, the current through the load does not immediately fall to zero but decreases approximately linearly with time, as the inductor's magnetic field collapses, and the current flows in a freewheeling diode. Thus, the input voltage pulses applied across the load by the power converter resembles a jagged current waveform. The variation in output current is typically known as the current ripple.
Current ripple is generally undesirable because it wastes energy in the inductor and may cause unwanted pulsations in the load. Current ripple produces undesirable EMI and mechanical vibrations, and generates harmonic losses in the motor. The harmonic current runs through the stator and rotor conductors and gives rise to harmonic copper losses, and the harmonic flux confines itself to the surface of the cores and creates harmonic core losses in the stator and rotor teeth. The higher the current ripple, the lower the quality of the current waveform of an induction machine.
Current ripple is primarily due to the harmonics contained in the PWM voltage waveform. For a given DC bus voltage, the current ripple is mainly dependent on the switching frequency of the power converter and the transient or leakage inductance of the machine. Equation 1 represents the machine current ripple factor as a ratio of the machine peak fundamental phase current.
                              k          ripple                =                              1                          I              max                                ⁢                                                                      V                                      D                    ⁢                                                                                  ⁢                    C                                                  ⁢                                  T                  s                                                            4                ⁢                                                                  ⁢                                  L                  σ                                                      .                                              Eqn        .                                  ⁢        1            Imax is the peak fundamental phase current (A), VDC is the DC bus voltage (V), TS is the switching period (s), and Lσ is the machine transient inductance (H).
If the leakage inductance of a phase of the induction machine is proportionally large, the current ripple will ordinary be low regardless of the switching frequency of the switching devices in the power converter. Thus, the higher the leakage inductance of an induction machine, the more closely the output current waveform resembles an ideal waveform. Accordingly, induction machines are typically designed to have a high enough leakage inductance to maintain a low current ripple.
However, designing a machine to have a high leakage inductance has a number of drawbacks including a reduced machine efficiency and a reduced machine flux-weakening capacity. Further, an increased leakage inductance injects harmonics via line notching in the power system.
For a number of applications, the leakage inductance associated with a particular phase is proportionately low. This low leakage inductance means that, for the same carrier frequency, the current ripple will be much higher. High current ripples inject potentially unacceptable amounts of noise into the motor. If the current ripple is high enough, this can cause the output to operate outside of its specified characteristics. In the case of an electric motor, for example, increased current ripple results in decreased control over torque and speed of the motor. Where the current ripple is high enough, the motor can fail altogether because it is not operating within its proper current range.
An extended flux-weakening capability is an important characteristic for an induction machine that is used, for example in a traction application, where it is desirable to operate the drive and induction motor over a wide speed range with a constant or near constant output power for speeds above the induction machine corner point. Operation at motor speed below the corner point is often referred to as operation in the “constant torque” region, while operation above the corner point of the induction machine is often referred to as operating in a “constant power” mode of operation. In general, a low-leakage inductance induction machine design allows “constant power” mode operation over a range of speeds in excess of 3 times the base speed of the induction machine.
It would therefore be desirable to design an induction machine that maintains a quality current waveform and that operates with a low leakage inductance to increase machine efficiency and extend the machine's flux-weakening capability.